The present invention relates generally to a serial printer, and, in particular, to a printer provided with a dot type printing head having a plurality of dot forming elements arranged as an assembly of dots to print characters and figures.
In a conventional serial printer, the dot type printing head is constructed with a large number of dot forming elements to achieve a high quality character printing and high printing speed. There are two types of arrangements of dot forming elements in a single printing head as set forth in the prior art. Referring specifically to FIG. 1(a), a first arrangement including a printing head having forty eight (48) dot forming elements is shown. When the printing head is mounted on a carriage, a four row dot forming element chain is formed by vertically arranging twelve (12) dot forming elements in four rows. Additionally, the twelve (12) dot forming elements are spaced at two dot intervals and are arranged to be vertically shifted by two dots between neighboring dot forming element chains.
Referring specifically to FIG. 2(a), a second arrangement is shown having four units. The four units are constructed by vertically arranging twelve (12) dot forming elements in four rows at two dot intervals. The neighboring first and second units and the third and fourth units are arranged to vertically align the dot forming positions. The dot forming positions are selected to vertically shift bit by bit between both the first and second units and the third and fourth units respectively.
In the first arrangement of the prior art, the dot forming positions of each unit are vertically shifted by 1/2 dot. Therefore, the dots are vertically arranged and interposed achieving an overlap of 1/2 dot as shown in FIG. 1(b). Accordingly, a fine dot pattern is produced with a high printing quality.
Further, the first arrangement of the prior art provides a printer having a draft printing mode with high printing speed, but inferior printing quality in addition to a normal printing mode with high printing quality. However, the disadvantages occur when a horizontal line is printed. When a horizontal line is printed by two units either the first and second units or the third and fourth units, the neighboring dots are vertically and alternately shifted by 1/2 dots so that character quality is degraded as shown in FIG. 1(c).
In the second arrangement of the prior art, two units are provided for printing the position of a character or figure. Therefore, since the assembly of dots are arranged in a straight line, high printing speed can be performed while at the same time a horizontal line can be printed with a high printing quality. However, since the dot forming elements are arranged at one dot intervals, a mutual interposition of dots can not be vertically performed as shown in FIG. 2(b). Therefore, the print quality is degraded in the normal printing mode as shown in FIG. 2(c).
Hence, the prior art discloses serial print heads which degrade the print quality in either the normal printing mode or the draft printing mode.
Accordingly, it is desired to provide an improved dot head for a serial printer which at a high printing speed can produce a high printing quality in both the normal printing mode and the draft printing mode.